One thing that comes to mind is to add a retention mechanism so that if it does back out, it doesn't get lost. Possibly trying machining a small hole through the knob to add a small loop of lockwire, then fasten a velcro belt to the tower, and add a small string with a hook at the end to hook onto the lockwire.

Obviously it doesn't solve the backing out issue, but it does prevent lost knobs while you work on making them stay tight.